This invention relates to a flue gas desulfurizer to be added to boiler equipment or the like, and to boiler equipment for utilizing the recovered heat in lower heat utilization equipment.
This invention also relates to thermal electric power generation equipment provided with a steam extraction feedwater heater for the steam turbine and with a flue gas desulfurizer.
Generally, a flue gas desulfurizer is used to remove sulfur oxides (SO.sub.x) and other gases present in flue gas from boiler equipment and discharge the resulting clean gas into the atmosphere.
FIG. 5 is a schematic view showing the typical construction of boiler equipment provided with such a flue gas desulfurizer and the temperature and moisture content of flue gas at various points.
Flue gas discharged from a boiler 1 is freed of nitrogen oxides (NO.sub.x) in a denitrator 2 attached to boiler 1, passed through an air heater (AH) 3 and the heat recovery section 4 of a gas-gas heater (GGH), and introduced into an electrostatic precipitator (EP) 5 where dust such as fly ash is removed therefrom. Then, by means of an induced draft fan (IDF) 6, the flue gas is introduced into a desulfurizer 7 where sulfur dioxides (predominantly sulfur dioxide) are removed therefrom. Thereafter, the flue gas is passed through the reheating section 8 of the gas-gas heater (GGH) and then discharged from a stack 10 into the atmosphere by means of a back-up fan (BUF) 9.
As the flue gas desulfurizer 7 used for this purpose, one of the wet absorption type in which flue gas is brought into a gas-liquid contact with an absorbent slurry so as to absorb and remove sulfur dioxide present in the flue gas is widely used in recent years because of its high degree of desulfurization and the like.
The temperature of the flue gas is typically about 135.degree. C. just behind air heater (AH) 3 and, when coal is used as the fuel, its moisture content is about 8%. Immediately before the inlet of flue gas desulfurizers 7, the temperature of the flue gas is lowered to about 90.degree. C. as a result of cooling in heat recovery section 4, and the like. In flue gas desulfurizer 7, the temperature of the flue gas is further lowered by contact with an absorbent slurry and, in the case of a coal-fired boiler, typically reaches about 48.degree. C. So long as no particular heat recovery or the like is effected in flue gas desulfurizer 7, the temperature of the flue gas at the outlet of flue gas desulfurizer 7 depends on the flow rate and temperature of the introduced flue gas, the gas-liquid contact capacity of flue gas desulfurizer 7, and the like. Since this gas-liquid contact capacity and the like are preset according to the properties (such as sulfur dioxide concentration) of the flue gas, the temperature of the flue gas at the outlet of flue gas desulfurizer 7 eventually depends almost uniquely on the properties of the fuel. For example, this temperature is typically about 48.degree. C. when coal is used as the fuel.
Since water present in the absorbent slurry evaporates to saturation in flue gas desulfurizer 7, the moisture content of the flue gas at the outlet of flue gas desulfurizer 7 is equal to the saturation level at 48.degree. C. (i.e., about 11%). In the case of a coal-fired boiler of the 1,000 MW class, the amount of water evaporated in the absorption tower of flue gas desulfurizer 7 is about 75 t/h.
Finally, the flue gas is heated to about 90.degree. C. in reheating section 8 and discharged from stack 10.
It is practically impossible to effect additional heat recovery by lowering the temperature of the flue gas to less than 90.degree. C. in heat recovery section 4, though this improves the performance of electrostatic precipitator 5 itself. Specifically, if the temperature of the flue gas is less than 90.degree. C., there is a possibility that the saturation temperature will be locally reached to produce steam condensate. This moisten the ash recovered by electrostatic precipitator 5 and makes its reuse very difficult. Moreover, in order to provide corrosion resistance to steam condensate, an expensive corrosion-resistant material needs to be used for components such as the ducts extending from heat recovery section 4 to flue gas desulfurizer 7, and induced draft fan 6.
Furthermore, the temperature of the flue gas must be raised to about 90.degree. C. in reheating section 8. Its purpose is to prevent the corrosion of downstream equipment, prevent the flue gas discharged into the atmosphere from being converted to white smoke, and secure its diffusibility. Specifically, unless the temperature of the flue gas is raised to about 90.degree. C. or above, there is a possibility that steam condensate will be produced, and an expensive corrosion-resistant material needs to be used for components such as the ducts extending to stack 10, and back-up fan 9. Moreover, the flue gas discharged into the atmosphere tends to be converted to white smoke, and its desired diffusibility is not obtained. Consequently, when the temperature of the flue gas is so low that its satisfactory diffusibility is not obtained and the smoke does not rise high, the height of the stack itself needs to be increased so as to meet the emission requirements.
Meanwhile, in equipment using boilers (e.g., thermal electric power generation equipment), the fullest utilization of thermal energy produced in boilers is increasingly required in recent years from the viewpoint of the effective utilization of resources. Thus, it is desirable to operate heat utilization systems, such as greenhouses, heated swimming pools and district heating systems, by use of surplus heat. However, in spite of various improvements conventionally made to achieve higher efficiency, it has been very difficult to recover heat from boiler equipment.
Also in flue gas treatment systems, it is problematic from a practical point of view, as described above, to cool the flue gas to less than the current level, for example, on the outlet side of the boiler. Thus, heat recovery has conventionally been considered to be difficult.
Moreover, in flue gas desulfurizers, the absorbent slurry needs to be replenished with water in an amount which is evaporated and carried away by the flue gas, resulting in the requirement of a large amount of industrial water. At the same time, such steam increases the total amount of the flue gas, causing an increase in the power consumption of the back-up fan and the size of the ducts extending to the stack.
Furthermore, in the prior art, thermal electric power generation equipment has been repeatedly improved until now in order to achieve higher efficiency. Generally, such equipment includes a plurality of high-pressure, intermediate-pressure and low-pressure steam turbines and a large number of extraction feedwater heaters for heating boiler feedwater successively by use of steam extracted from various points of each turbine. However, it has been very difficult to further enhance the thermal efficiency in the complicatedly constructed steam cycle system of thermal electric power generation equipment.